Pancreatitis is a common medical disease with substantial morbidity and mortality. Severalwell-defined clinical conditions are associated with acute and chronic pancreatitis and include: alcohol ingestion, gallstone disease, drug exposures, and various metabolic, immunologic, and infectious causes. Irrespective of the cause, many of the pathophysiological processes in the generation of pancreatitis are common to alletiologies. The pancreas synthesizes and stores abundant enzymes and, should they become prematurely activated, are potentially destructive to the gland. Pancreatitis is believed to result from premature activation ofpancreatic enzymes within the parenchyma of the gland, leading to cellular autodigestion, subsequent inflammation, and ultimately tissue destruction. When the insults are prolonged or with repeated attacks of acute pancreatitis, chronic pancreatitis can result. With the recent identification of genetic mutations in the trypsinogen and pancreatic secretory trypsin inhibitor (PSTI) genes and their associations with hereditary and familial pancreatitis, respectively, it appears that a balance between activated proteases and endogenous trypsin inhibitors within the gland is critical for the prevention of pancreatitis. The principal investigator has preliminary data indicating that transgenic expression of PSTI in mice may protect against acute and chronic pancreatitis. The objective of the current proposal is to determine the role of endogenous trypsin inhibitors in the pancreas and evaluate their importance in pancreatitis. This will be accomplished by examining the hypothesis that endogenous pancreatic secretory trypsin inhibitor protects against pancreatitis by inhibiting intracellular trypsin activity. This hypothesis will be tested through the following specific aims: (1) determine the intracellular actions of PSTI in mice expressing the PSTI transgene; (2) examine the effects of PSTI expression on acute and chronic pancreatitis; (3) determine the mechanism by which PSTI protects against pancreatitis; and (4) examine the effects of reducing endogenous trypsin inhibitor activity on pancreatitis using PSTI knockout mice. It is expected that the results of these studies will establish the role for endogenous trypsin inhibitors in preventing pancreatitis, determine the mechanism by which trypsin inhibitors confer protection, and may ultimately lead to novel strategies for treatment or prevention of the disease.